Superoxide

Source: Wikipedia, the free encyclopedia.
Superoxide

Lewis structure of superoxide. The six outer-shell electrons of each oxygen atom are shown in black; one electron pair is shared (middle); the unpaired electron is shown in the upper-left; and the additional electron conferring a negative charge is shown in red.
Names
IUPAC name
Superoxide
Systematic IUPAC name
Dioxidan-2-idylide
Other names
Hyperoxide, Dioxide(1−)
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
487
KEGG
UNII
  • InChI=1S/O2/c1-2/q-1
    Key: MXDZWXWHPVATGF-UHFFFAOYSA-N
  • O=[O-]
Properties
O−2
Molar mass 31.998 g·mol−1
Conjugate acid
 Hydroperoxyl
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

In

free radicals that exhibit paramagnetism.[3] Superoxide was historically also known as "hyperoxide".[4]

Salts

Superoxide forms salts with alkali metals and alkaline earth metals. The salts sodium superoxide (NaO2), potassium superoxide (KO2), rubidium superoxide (RbO2) and caesium superoxide (CsO2) are prepared by the reaction of O2 with the respective alkali metal.[5][6]

The alkali salts of O−2 are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolved O−2 undergoes disproportionation (dismutation) extremely rapidly (in a pH-dependent manner):[7]

4 O−2 + 2 H2O → 3 O2 + 4 OH−

This reaction (with moisture and carbon dioxide in exhaled air) is the basis of the use of

Brønsted base, initially forming the hydroperoxyl
radical (HO2).

The superoxide anion, O−2, and its protonated form, hydroperoxyl, are in equilibrium in an aqueous solution:[8]

O−2 + H2O ⇌ HO2 + OH−

Given that the hydroperoxyl radical has a

pKa of around 4.8,[9]
superoxide predominantly exists in the anionic form at neutral pH.

Potassium superoxide is soluble in

aprotic solvents by cyclic voltammetry
.

Superoxide salts also decompose in the solid state, but this process requires heating:

2 NaO2 → Na2O2 + O2

Biology

Superoxide is common in biology, reflecting the pervasiveness of O2 and its ease of reduction. Superoxide is implicated in a number of biological processes, some with negative connotations, and some with beneficial effects.[10]

Like hydroperoxyl, superoxide is classified as

Complex III), as well as several other enzymes, for example xanthine oxidase,[11]
which can catalyze the transfer of electrons directly to molecular oxygen under strongly reducing conditions.

Because superoxide is toxic at high concentrations, nearly all aerobic organisms express SOD. SOD efficiently catalyzes the disproportionation of superoxide:

2 HO2 → O2 + H2O2

Other proteins that can be both oxidized and reduced by superoxide (such as hemoglobin) have weak SOD-like activity. Genetic inactivation ("knockout") of SOD produces deleterious phenotypes in organisms ranging from bacteria to mice and have provided important clues as to the mechanisms of toxicity of superoxide in vivo.

cataracts, thymic involution, haemolytic anemia, and a very rapid age-dependent decline in female fertility.[11]

Superoxide may contribute to the pathogenesis of many diseases (the evidence is particularly strong for

cataracts, muscle atrophy, macular degeneration, and thymic involution). But the converse, increasing the levels of CuZnSOD, does not seem to consistently increase lifespan (except perhaps in Drosophila).[11]
The most widely accepted view is that oxidative damage (resulting from multiple causes, including superoxide) is but one of several factors limiting lifespan.

The binding of O2 by reduced (Fe2+) heme proteins involves formation of Fe(III) superoxide complex.[12]

Assay in biological systems

The assay of superoxide in biological systems is complicated by its short half-life.

spin traps" that can react with superoxide, forming a meta-stable radical (half-life 1–15 minutes), which can be more readily detected by EPR. Superoxide spin-trapping was initially carried out with DMPO, but phosphorus derivatives with improved half-lives, such as DEPPMPO and DIPPMPO, have become more widely used.[citation needed
]

Bonding and structure

Superoxides are compounds in which the

free radicals that exhibit paramagnetism
.

The derivatives of dioxygen have characteristic O–O distances that correlate with the order of the O–O bond.

Dioxygen compound name O–O distance (Å) O–O bond order
O+2 dioxygenyl cation 1.12 2.5
O2 dioxygen 1.21 2
O−2 superoxide 1.28 1.5[14]
O2−2 peroxide 1.49 1

See also

  • Oxygen, O2
  • Ozonide, O−3
  • Peroxide, O2−2
  • Oxide, O2−
  • Dioxygenyl, O+2
  • Antimycin A – used in fishery management, this compound produces large quantities of this free radical.
  • Paraquat – used as a herbicide, this compound produces large quantities of this free radical.
  • Xanthine oxidase – This form of the enzyme xanthine dehydrogenase produces large amounts of superoxide.

References

  1. PMID 26875845
    .
  2. doi:10.1036/1097-8542.669650
  3. ^
    PMID 16978905
    .
  4. PMID 26875845
    .
  5. ISBN 0-12-352651-5
    .
  6. doi:10.1021/i360062a015
    .
  7. ISBN 0-471-84997-9
  8. doi:10.1063/1.555739
    .
  9. ^ "HO•
    2    : the forgotten radical Abstract"     (PDF). Archived from the original (PDF) on 2017-08-08.
  10. PMID 21151885
    .
  11. ^
    PMID 17640558.{{cite journal}}: CS1 maint: numeric names: authors list (link
    )
  12. ^ Yee, Gereon M.; Tolman, William B. (2015). "Chapter 5, Section 2.2.2 Fe(III)-Superoxo Intermediates". In Kroneck, Peter M.H.; Sosa Torres, Martha E. (eds.). Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. Vol. 15. Springer. pp. 141–144.
    PMID 25707468
    .
  13. ^
    S2CID 40487242
    .
  14. doi:10.1107/S0365110X55001540
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Superoxide&oldid=1219955239"